by Antonio Castro <acastro(at)ctv.es> About the author: I am computer scientist and have had the opportunity to work with several Unix flavors. Furthermore I have done different tasks from software design to system administration. Translated to English by: Miguel A Sepulveda <Miguel.Sepulveda(at)disney.com> Content: Introduction Selection of Disks for a RAID Characteristics of a SCSI system Types of RAID How to Install a RAID0 For more information:

Installation and Configuration of a RAID System Abstract: RAID (Redundant Array of Inexpensive Disks) consists of a series of systems to organize several disk drives into a single entity that behaves as a single virtual drive but making the various disks work in parallel ,thus improving the access performance and saving the information stored from accidental crashes. ACKNOWLEDGEMENTS I will always be grateful to the whole LinuxFocus team that has collaborated often times from a non visible position to improve the aspect of the article as well as to the translators. On this occasion I must mention a person in particular. This article would not have been possible without the help of Luis Colorado, who, in email after email shared with me his knowledge on RAID systems. Thanks Luis. _________________ _________________ _________________

Introduction

There are also other RAID implementations based of cards that allow a user to manage several identical disks devices as a RAID thanks to a simple Z80 chip and on board software. Under these specifications it is not possible to claim that this solution would give better efficiency than a Linux based solution.

Implementations based on controller cards are expensive and also force the user to purchase only identical disk devices. Linux, on the other hand, given the appropriate device drivers could use some of these cards, but that would not be an interesting solution since Linux allows a free software based solution, equally efficient that avoids the expensive hardware alternatives.

By contrast using multiple disk devices on the same IDE controller card means that these devices will never be able to be accessed simultaneously. It is a pity that SCSI disks are still much more expensive than its IDE counterparts. The software solution for a Linux RAID system is equally efficient (if not more) than those based on special cards and of course cheaper and more flexible in terms of the disk devices permitted.

While in a SCSI bus a device can be dumping data to the bus while another is retrieving it, on an IDE interface a disk is first accessed and the other afterwards.

Selection of Disks for a RAID

It is necessary also to take into account that the Linux system must start from a non RAID disk device, and of small size so that the root partition is relatively free.

Characteristics of a SCSI system

IDE devices have file devices under Linux named /dev/hd..., to SCSI devices correspond /dev/sd..., and to metadisks there will be /dev/md.. after compiling the kernel with the options specified later. Four such devices should be present:

brw-rw----   1 root     disk       9,   0 may 28  1997 md0
brw-rw----   1 root     disk       9,   1 may 28  1997 md1
brw-rw----   1 root     disk       9,   2 may 28  1997 md2
brw-rw----   1 root     disk       9,   3 may 28  1997 md3

Types of RAID

• RAID0 (Stripping mode)
  In this mode, all the disk devices are organized alternatively so that blocks are taken equally from all disks, alternatively, in order to reach higher efficiency. Since the probability of finding a block of a file is identical for all disks, there are force to work simultaneously thus making the performance of the metadisk almost N times that of a single disk.
• RAID1
  In this mode, the goal is to reach the highest security of the data. Blocks of data are duplicated in all physical disks (each block of the `virtual' disk has a duplicate in each of the physical disks). This configuration provides N times the reading performance of a single device, but it degrades writing operations. Read operations can be organized to read N blocks simultaneoulsly, one from each device at a time. Similarly when writing 1 block it has to be duplicated N times, one for each physical device. There is no advantage in this configuration regarding storage capacity.
• RAID4
  (Note: the RAID2 and RAID3 types are obsolete).
  

  In this mode the ultimate goal is to balance the advantages of the type RAID0 and RAID1. Data is organized mixing both methods. The physical 1 to N-1 are organized in striping mode (RAID0) and the Nth stores the parity of the individual bits corresponding to blocks 1 to N-1. If any of the disks fails, it is possible to recover by using the parity information on the Nth hard disk. Efficiency during read operations is N-1 and during write operations is 1/2 (because writing a data block now involves writing also to the parity disk). In order to restore a broken hard disk, one only has to re-read the information and re-write it (it reads from the parity disk but it writes to the newly install hard disk).

• RAID5
  This type is similar to RAID4, except that now the information of the parity disk is spread over all the hard disks (no parity disk exists). It allows to reduce the work load of the parity disk, that in RAID4 it had to be accessed for every write operation (now the disk where parity information for a track is stored differs for every track)

If the reader cannot use identical disks take into account that RAID systems always work with identical blocks of information. It is possible that the slow hard disks will be forced to work harder, but in any case the RAID configuration will still yield a better performance. The increase of performance on a RAID system that is properly configure is truly spectacular. It is almost true to say that the performance increases linearly with the number of hard disks in the RAID.

How to Install a RAID0

RAID0 has no redundancy but consider that to have redundancy it is advised a large number of disks in order not to waste too much disk capacity. Wasting a whole disk when we only have three is a waste. Furthermore, it does not cover all the possible cases of information lost but only those due to physical deterioration of the hard disks, a very uncommon event. If 10 hard disks were available, then using one for parity control is not so much of a waste. On a RAID0 having a disk failure on any of the disks means losing all the information stored in all the physical disks, consequently we recommend an appropriate policy of backups.

The first step to take is adding the appropriate drivers to the kernel. For Linux 2.0.xx RAID the options are:

   Multiple devices driver support (CONFIG_BLK_DEV_MD) [Y/n/?] Y
      Linear (append) mode (CONFIG_MD_LINEAR) [Y/m/n/?] Y
      RAID-0 (striping) mode (CONFIG_MD_STRIPED) [Y/m/n/?] Y

        -mdadd
        -mdrun
        -mdstop
        -mdop

For kernels 2.1.62 and higher there is a different package called 'RAIDtools' that permits to use a RAID0, RAID4 or RAID5.

In the following example we illustrate how to define a RAID0 metadisk that uses two hard disks, more specifically /dev/sdb1 and /dev/sdc1.

Once the metadisk is formatted it should not be altered under any cricunstance or all the information in it would be lost.

mdadd -a
mdrun -a

mke2fs /dev/md0

mkdir /mount/md0
mount /dev/md0 /mount/md0

For Example:

HD 4.2Gb SCSI

HD 2Gb SCSI

#######</etc/fstab>################################################
# <file system> <mount point>  <type>  <options>     <dump>  <pass>
/dev/hda1       /               ext2    defaults       0       1
/dev/hda2       /mnt/hda2       ext2    defaults       0       2
/dev/md0        /mnt/md0        ext2    defaults       0       2
proc            /proc           proc    defaults       0       2
/dev/hda3        none           swap    sw,pri=10
/dev/sdb1        none           swap    sw,pri=10
/dev/sda3        none           swap    sw,pri=10

#########</etc/mdtab>#######################################
# <meta-device> <RAID-mode> <DskPart1> <DskPart1> <DskPart1>
/dev/md0         RAID0,8k    /dev/hda4  /dev/sda4 /dev/sdb2

We should try to make all disks (partitions) in the RAID of approximately the same size because large differences will decrease the RAID performance. Small differences are not significant. We use all the space available so that all the data from the disks that can be entangled is and the remaining data remains free.

Mounting several IDE disks on a single RAID is not very efficient, but mounting an IDE with various SCSI works very well. IDE disks do not allow concurrent access, while SCSI disks do.

For more information:

• mdutils COmes with documentation
• mini-howto Multiple-Disk
• The Multiple Disk Layout mini-HOWTO Homepagewww.nyx.net/~sgjoen/disk.html

2002-11-03, generated by lfparser version 2.34